(1) Field of the Invention
The present invention relates to a multi-layer surface-treated steel plate having a zinc-containing layer, and the present invention provides a surface-treated steel plate excellent in the rust proofness, paint adhesion and corrosion resistance of the coating.
(2) Description of the Prior Art
As the surface-treated steel plate to be employed as a substrate to be coated in the field of production of household electric appliances or construction materials, there have broadly been used products obtained by forming a phosphate treatment layer or chromate treatment layer on a zinc-deposited steel plate. In a chromate-treated zinc-deposited steel plate, a good corrosion resistance can be obtained owing to the passivating action of chromium, but there are problems concerning the toxicity of chromium and the waste water treatment. The phosphate treatment provides an undercoating having excellent properties, but in order to obtain a sufficient corrosion resistance, the chromic acid treatment should be performed as the post treatment and because of this post treatment, the same problems as encountered in the chromate treatment arise and furthermore, a problem of disposal of sludges formed in large quantities arises. Moreover, surface-treated steel plates obtained by either the chromate treatment or the phosphate treatment are still insufficient as substrates to be coated in the corrosion resistance of the coating, the paint adhesion and the degreasing resistance. Accordingly, development of a surface-treated steel plate having excellent, well-balanced properties as a substrate to be coated has been desired in the art.
As the surface-treatment method for solving the foregoing problems, there has been proposed a method using a silicate composite composed of silica and an acrylic copolymer (see Japanese Patent Publication No. 34406/79), and some improvements of this methods have been proposed in Japanese Patent Application Laid-Open Specifications No. 77635/79 and No. 62971/80. However, when this silicate composite (hereinafter referred to as "organic composite silicate") is applied to a zinc-deposited or zinc alloy-deposited steel plate, the paint adhesion is improved over the paint adhesion attained by the existent chromate treatment or phosphate treatment, but the corrosion resistance in either the uncoated state or the coated state is insufficient and it is desired to further improve the corrosion resistance.